The present invention relates generally to span locks for drawbridges, and more particularly to a novel and improved energy absorbing span lock system suitable for use in drawbridges.
Span locks on drawbridges serve as shear connectors for securing the outer ends of movable leaves from vertical deflection such as occurs during passage of vehicular traffic, and for preventing the bridge from accidentally opening. As applied to a double-leaf bascule bridge, the locks couple the outer ends of the leaves in vertical alignment where they meet and transfer the shear loads from one leaf to the other.
Several types of locks for these purposes are well-known. For example, U.S. Pat. Nos. 685,768 to C. L. Keller and 689,856 to E. D. Cummings provide interlocking members for vertically aligning the outer ends of bascule bridge leaves where they meet, and for transmitting shear stresses from one leaf to the other. The bascule bridge in U.S. Pat. No. 2,610,341 to H. H. Gilbert utilizes a bar lock consisting of self-aligning members on one leaf registering with retractable shear pins on the other leaf. In German Patent No. 929,130 to K. Seegers, the drawbridge has a span lock in which spring-loaded locking levers on one leaf snap into place on either side projection on the other leaf as the span closes.
Bar-type and jaw-type span locks have been particularly favored because of their reliability. The bar-type lock utilizes forged steel, machine-finished bars slid within guides and sockets on the outer or meeting ends of the drawbridge. The bar lock is especially susceptible to wear requiring frequent interruptions of bridge service for time-consuming repair or replacement of parts. Some bar locks have shims in the guides and sockets for adjusting the positions of the openings and for controlling clearance with the bar. Even so, service interruptions are still necessary for adjustments and shim replacement. Where shims or other means for adjustment are not provided, complete disassembly and removal of span lock components are usually required.
The jaw-type span lock provides extendable jaws on one bridge leaf which rotate on eccentric pins in a vertical plane into a socket on the other bridge leaf. The vertical clearance between the jaws and the socket is usually adjusted by rotating the eccentric pins. Generally, pins are difficult to move after long use and exposure to the environment, and are not in readily accessible locations.
Vertical clearance in each of these span lock systems is critical. Vertical clearance in the range of 0.020 inches is preferred yet, after a short service time, it is not uncommon for the clearance to become excessive. In some instances, vertical movement of one end of a meeting bridge end relative to the other increases as much as 1.5 inches or more. In addition to the risk of vehicular accidents and personal injury, excessive vertical clearance is insidious and will continue to increase and manifest damage or failure as well to structural members or operating machinery due to greater shock loading.
Ideally, the vertical opening in either system should be equal to the height of the bar, or distance between the jaws, so that substantially no clearance exists upon contact, and that shear loads are smoothly transmitted from one member to the other. However, minimizing the vertical clearance is not easily achieved. In one bar-type system the guides and sockets are equipped with upper and lower shoes urged against the bar by tapered wedges. As wear occurs, the vertical clearance between the bar and the shoes can be reduced by adjusting the wedges. The wedges, however, are not a significant improvement over guides and sockets fitted with shims. Both designs require frequent adjustment to insure proper alignment.
Another known bar-type span lock design is self-adjusting. It utilizes a tapered bar slidable in a guide and a socket in respective leaves of the bridge. As the bar or socket wears, the bar is urged further into the socket to take up the wear. The position of the bar in the socket is controlled by limit switches. This design accommodates wear in the socket of one leaf, but does not correct for wear in the bar guide.
While the above-described designs have unique advantages over prior concepts, they do not address the problem of repetitive shock loads experienced by span locks in drawbridges due to heavy traffic.